Variable vacuum tube resistor



April 14, 1936. F. E. TERMAN VARIABLE VACUUM TUBE RESISTOR 2 Sheets$heetl Filed May 25, 1951 Wm WM NT 5 a H m m ATTORNEY April 14, 1936. F,TERMAN' 2,037,292

VARIABLE VACUUM TUBE RESISTOR Filed May 25, 1931 2 Sheets-Sheet 2INVENTOR FREDERICK E. TERM/4N. w r

ATTORNEY Patented Apr. 14, 1936 UNITED STATES PATENT OLFFICE 1 Claim.

Myinvention relates to variable resistance elements, and particularly toelements whose resistance may be varied cyclically at any desiredfrequency.

In my copendingapplication, Serial No. 489,917, filed October 20, 1930,I have shown a method of producing side-band frequencies, in the absenceof a carrier frequency, by passing a modulating current through animpedance element which varies cyclically at a carrier frequency. Insaid application I have shown and claimed one type of vacuumtuberesistance element which may be varied in the manner described. Thisapplication is directed to another general method of obtaining thedesired result.

Among the objects ofthis invention are: first, to provide a cyclicallyvariable resistance element which is bilaterally symmetrical; second, toprovide a method of operating a multi-element vacuum tube of ordinaryconstruction to provide a resistance element of the typedescribed;third, to provide specific circuits wherein such resistance elements maybe used with maximum advantage; and fourth, to provide a general method5 whereby tubes of ordinary construction may be utilizedas symmetricalvariable resistors in alternating current circuits.

Other objects of my invention will be apparent or will be specificallypointed out in the description forming a part of this specification, butI do not limit myself to the embodiment of my invention, hereindescribed, as various forms may be adopted within the scope of theclaim.

Referring to the drawings:

Figure 1 is acircuit diagram showing the use of a special threeelectrode vacuum tube as a cyclicall y variable resistance element whichis symmetrical with respect to current flow.

Figure 2 is a. circuit diagram showing the. use of the ordinary threeelectrode vacuum tube, as a. variable. asymmetric resistance element.

Figures: 3 and 4 are. characteristic curves of different three electrodevacuum tubes, illustrating the portion of the characteristics whereinsuch tubes: may be used as. variable resistors.

Figure- 5. isa circuit diagram illustrating the use. of a. tetrode as, avariable resistor.

Figure 6 illustrates another circuit by which a tetrode may be used .inaccordance with this invention. 9

Figure 7 illustrates a, push-pull circuit, wherein two triodes are usedto give. asymmetrical variable resistance.

. 5 Figures. 8. and, 9.- show two. circuits utilizing triodes in a, backto-back connection for. giving a symmetrical variable resistance.

Figure 10 illustrates a circuit wherein a, single double filament. tube.is utilized to give substantially the same characteristic as the backto-5 back connection of Figure 8.

Figure 11 illustrates. the use of a single, tube having two. separateplates ina circuit similar to that of Figure '7, wherein two tubes areused.

Under the conditions in. which it is, ordinarily 10 used, a vacuum tubecannot truly be said to have any fixed value which may be assigned asits plate resistance. That is, although there is a value of platecurrent corresponding to any given value of plate and grid voltages, iithe 15 plate voltage be varied the plate current does not alter inlinear relation, or, if plate and grid voltages be variedsimultaneously, the tube does not act as a simple resistor whose. platecircuit varies in resistance in accordance. with the vary- 20 ing gridpotential.

I have found, however, that if. the plate or other output electrode beoperated at a potential such that it is not the determining factor inestablishing the total space current within 5 the tube, the latterbeing, at any control electrode potential, substantially independentotthe potential on the output electrode, and if another electrode withinthe tubebe varied to change the total space current, the distributionof'current 30 output. electrode current.

In general, this resistance is asymmetric. That is, it. isinfmite forpotentials in one direction and finite for potentials in the oppositedirection. This condition is not necessarily undesirable, but where asymmetrical resistor is. desired it may be achievedby connecting twotubes-in opposite relation, either push-pull? or bacletc-back so as togive equivalent results on both halvesof the waves. Furthermore, tubesmay be constructed having either two: filaments; or two plates, which.may; be. symmetrically connected tota circuit. in

substantially the same manner as two tubes. 5 5

The various figures show in detail various methods of carrying out theinvention thus described in general terms. In each of the circuits thusindicated a source I of alternating current power is connected throughthe primary 2 of a transformer to what would ordinarily be consirderedthe output terminal of a vacuum tube which acts as a cyclically varyingresistance element. There will then be induced in a transformersecondary 3 a current of side-band frequency which may be carried by theoutput leads 4 and applied to any desired amplifier or other utilizationdevice.

As used throughout this specification the term side-band frequencyrefers to a. frequency (ftp). where f is a relatively high carrierfrequency, and p is the relatively low modulating frequency supplied bythe source 1.

The circuits shown differ in the details of the vacuum tube resistanceelement, perhaps the simplest being that shown in Figure 2, wherein theleads 6 and I connect from the plate 8 of the tube to the power source Iand from the cathode 9 to the transformer secondary 2, re-

spectively. V

The tube III, is, in this case, the ordinary triode or three electrodevacuum tube. A positive potential is supplied by the battery I I to thecontrol electrode or grid I2 of the tube, and in series with the-batteryis a source I3 of alternating current potential of carrier frequency. Asimilar source I3 of carrier frequency potential is common to all of thecircuit diagrams, and exercises a similar function in every case.

In order to maintain the conditions set forth above in the threeelectrode vacuum tube, the potential from the power source I must below,

i. e., in the neighborhood of zero. Moreover, the

battery II must be arranged to place a positive potential upon the grid.Under these circumstances, the total space current flowing dependsalmost wholly upon the grid potential, whereas the proportion of thisspace current which flows to the plate is a function of the platepotential. These results may be seen from the characteristic curves ofFigs. 3 and 4, which were taken with different types of tubes operatingunder the conditions set forth.

It will be noted that in each of these figures the characteristic curvescorresponding to different grid voltages meet or coincide at a point I4,at which zero plate current flows. With certain tubes, this point willalso correspond to zero plate voltage; with other tubes, a biasingbattery I5 may be supplied in the plate circuit, to render the point ofzero plate current coincident with zero voltage of the power source I.

In Figure 4 the curves also intersect at the point I I, and this also isa suitable operating point,

although a direct current component may be present in this case whichmust be taken into consideration. This D. C. component introduces acarrier frequency component in the output circuit, which may be balancedout if undesirable.

The curves shown in these two figures are characteristic, and willdiffer little in form as between tubes of varying size, although themagnitudes of the quantities will vary considerably. The voltagecorresponding to the point I4 will vary little with the type of tube,since the negative potential corresponding to zero plate current is afunction of the initial velocity of emission from the cathode, anddepends on the nature of its surface and the temperature at which it isoperated. The current values, and corresponding 'vention utilizing atetrode 25.

grid potentials will vary with the type of tube and particularly withthe dimensions and spacing of the electrodes. Thus, a positive grid biasof 5 to 10 volts may be the maximum permissible with a small tube,whereas, a large tube with widely spaced elements may require volts ormore positive bias for best operation.

In the circuit of Figure 2, the effective resistance of the tube isinfinite on one-half of the cycle. This is a disadvantage in someservices, which may be overcome by utilizing a special tube as is shownin Figure 1. In this case, the'leads I5 and 'I' each connect to afilament I6, a single grid II being arranged between the two filaments.A carrier potential source I3, in series with the battery I8, connectsto the center of a network which is arranged to by-pass the carrierfrequency to both sides of the modulating frequency circuit, i. e., theleads 6' and I. This network is shown as comprising two inductorsI9,-I9', which offer a high impedance to the modulating frequency fromthe source I when bridged by the condensers 2U, 20', the latter elementsserving to by-pass the carrier frequency.

It will be seen that in this arrangement each of the filaments in turnacts as a cathode while the other filament performs the anode function.Positive potential on the grid I'I causes a substantially constant spacecurrent to flow from each filament at any given grid potential, but thewith the tube In of Figure 2.

In Figure 5 is shown a modification of my in- In this case the leads 6and I connect to the filament and plate of the tube respectively asbefore. The screen grid 26 of the tube is maintained at a positivepotential by a battery 21, but in this case the grid 28 is maintainednegative by a battery 29. The total space current is independent of thepotential of the-plate 30, being a resultant of the potential of thegrid 28 and the screen grid 26, and

is controlled by variations of potential of the con.-

trol grid 28.

The effect of this arrangement is very similar to that of Figure 2, butconsiderably higher potentials may be used on the plate, and thereforemore power may be absorbed in the circuit with a greater amount ofside-band frequency power withdrawn from the leads 4.

In Figure 6, the tube 25' acts in much the same way as the tube 25 ofthe preceding figure. In

this case, however, the carrier frequency potential source I3 isconnected in series with the screen grid 26 and battery 21, while theinner grid 28' is held at a constant negative potential by means of thebattery 29'. V

Figure 7 illustrates a push-pull method of obtaining a symmetricalresistance circuit. In this case the leads 6' and I connect to theprimary of a modulating frequency transformer 35. The two ends of thesecondary 36 of this transformer connect with the plates 31, 31' of thetwo triodes 38, 38, while the center tap on the secondary connects tothe filament 40, 40. The battery 4| places a positive potential uponboth grids 42, 42, superposed upon which is the carrier frequencypotential from the source I3. The two tubes 38, 38' vary in' resistancetogether, each tube presenting a finite resistance during one-half ofthe cycle of the modulator or power source I. The

changes in impedance vary the effective impedance of the transformer asseen from the source I, to effect the desired results. A biasing battery43 may be used, like the battery I5 of Figure 2, to bring the platepotential to the optimum operating point. I

In Figure 8 each of the two tubes. 45 and 45 is connected to themodulating power source circuit in substantially the same manner as thesingle tube of Figure 2, the difference being that the two tubes havetheir filaments connected to opposite sides of the circuit. The carrierfrequency potential is applied to the grids from the source I3 through atransformer having a single primary 46 and separate secondaries 41, 4?.Separate batteries 48, 48 are also provided for keeping the grids 49,49' positive. The transformer arrangement is used to keepthe potentialof the modulating source I from the two grids.

Figure 9 shows a modification of the back-toback circuit wherein themodulating frequency is kept from the grid of the tube and the carrierfrequency is kept from the plate, without actual physical separation ofthe circuits by means oi a transformer, as in the case of Figure 8. Inthis case high frequency choke coils 55, are inserted in the circuitbetween the tubes 56, 56' and the carrier potential source I3. Blockingnetworks 51 and 51' are inserted between each side of the circuit 6', Iand the control grids 58, 58' to prevent the modulating frequencypotential from reaching the grid. A similar blocking network 60 isinserted in series with the carrier potential source I3 and thefilaments of the two tubes. Except for the'method of separating thepotentials, this circuit behaves in the same manner as that of Figure 8.The constants of the blocking networks 51, 51' and 60 depend of courseupon the modulating frequencies used. The ba teries (SI and 6| keep thetwo grids at positive potentials as in the preceding cases.

A peculiarity of this circuit is that the resistances of tubes 56 and 56vary in opposite senses with the potential from the carrier frequencysource. The result is that there is a change in phase in current in theresistor circuit, as compared with the output of the other circuitsshown, as the current passes through zero. The same effect may beobtained from the circuit of Figure 8, and also from thecircuit next tobe described, if the transformer secondaries connecting source I3 to thetubes be connected to swing the grids in opposite phase. Whether this isto be avoided or not depends upon the use to be made of the resultantcurrent; in some cases the phase reversal may be advantageous.

Figure 10 shows another modification of the circuit of Figure 8 whereinthe two tubes are replaced by a single tube somewhat similar to thatshown in Figure 1, but having two grids 66 and 66 positioned between thetwo filaments 61, 61'. As in the modification of Figure 1, each filamentacts alternately as a cathode and as an anode. The carrier potentialsource I3 supplies the two grids through a transformer comprising oneprimary 58 and two secondaries 69, 69, as in Figure 8, and separatepositive biasing batteries I0, III are used.

Figure 11 shows a single tube modification of the push-pull circuitshown in Figure '7. The modulating power source leads to primary 15 of atransformer whose effective resistance changes with changes in the loadupon its secondary. The single filament I6 is connected to a center tapon the transformer secondary TI. The two ends of the secondary areconnected to two separate plates I8, 18 within the single tube. The gridI9 may either be a single structure surrounding the filament, or twoseparate grids, one on either side of the filament, connected together.The source I3 is connected through the positive biasing battery to thefilament.

It will be understood that the three electrode structures shown in mostof these circuit modifications may be replaced in practically any of thecircuits by tetrode or pentode tubes operating upon the same principle.Three electrode tubes, 7

or at least tubes operating upon the ordinary three electrode principle,are shown in the various circuit modifications merely because these leadto the simpler structures, but the method of using four or five elementtubes in similar structures will at once be apparent, the resultantcircuits being related to those shown in the same manner in which thecircuits of Figures 5 and 6 are related to the simple circuit of Figure2.

I claim:

The method of operating a tetrode having an anode, cathode and two gridsas a resistor whose value is determined by the potential of one of theelectrodes thereof which comprises applying an alternating potentialbetween the cathode and plate of said tetrode, applying a fixed negativepotential to the inner grid of said tetrode, and varying the positivepotential of the outer grid to vary the effective resistance betweensaid'cathode and said plate.

FREDERICK E. TERMAN.

